Different techniques to characterize frequency translating devices (FTD) can presently be implemented using commercially available vector network analyzers (VNA). Frequency translating devices are among others mixers where apart from the input and output port also the local oscillator (LO) port can be accessed to apply externally a local oscillator signal, mixers with integrated local oscillator and as such without access to the LO port, in phase/quadrature (IQ) modulators and demodulators or any other system or component arranged for producing a frequency translated signal. Such characterization is aimed to measure the reflection factors, leakage paths and frequency conversion factors of the FTD between all relevant ports, depending on accessibility.
Solutions have been proposed wherein a two-tone signal (i.e. two tones at a different frequency) is used to determine the frequency convertor terms (mainly group delay) for a FTD with integrated LO (hence, without access to the LO signal) in a differential way. By stepping the two-tones in frequency, with each time an overlapping frequency, it is possible to determine the frequency up and down converter terms separately, except for an unknown constant phase. This technique suffers from the fact that the mismatches of the FTD and of the measurement system are not determined and that the characterization does not include the dependency on at least the phase of the LO signal. Also the power dependency on the LO signal is missing, which is less important for this type of FTDs as the power is fixed for an integrated LO.
In patent document U.S. Pat. No. 6,690,722 a method is proposed to determine the frequency converter terms by measuring the reflection factor of the FTD with integrated LO with a one port network analyser, while applying known impedances at the other port of the FTD, which has been extended with a filter for image rejection to make the method work. This allows deriving the input reflection factor as such, which contains the combination of the up- and downconversion factor, but which is distorted by the reflection of the image rejection filter and the output reflection factor of the filter. The method needs to assume reciprocity between up conversion and down conversion to determine the frequency conversion factor.
Other possible techniques employ a mixer in addition to the FTD under test in the receiving path to convert to the same IF frequencies at all network analyzer ports. A mixer considered as ideal (also referred to as “golden” mixer in the technical literature) is used as reference and the FTD under test is measured and compared to that reference mixer. Such an approach is adopted e.g. in U.S. Pat. No. 6,448,786 and U.S. Pat. No. 7,248,033.
In U.S. Pat. No. 6,064,694 an alternative technique is presented based on the use of three mixers and results in the FTD characteristics assuming reciprocity between up and down conversion. Access to the LO is required for this method.
In the paper ‘Multi-tone, Multi-port, and Dynamic Memory Enhancements to PHD Nonlinear Behavioral Models from Large-signal Measurements and Simulations’ (Verspecht et al., IEEE/MTT-S International Microwave Symposium, June 2007, pp. 969-972) it is explained that using X-parameters a model can be constructed for a mixer that takes into account the non-linear effect introduced by the LO drive signal and by the main tone applied to the input port of the mixer, which also acts as a large signal at large enough amplitudes. To extract this model, many more measurements need to be carried out in order to take into account the amplitude of both large signals and their phase differences.
While for linear devices the S-parameters provide a straightforward approach to characterize the device for arbitrary input signals or incident waves, none of the known techniques provides this functionality to characterize a mixer, or a frequency translating device (FTD) in general, for its most important or primary behaviour. Hence, there is a need for a primary model for a FTD, i.e. a model that describes at the same time all FTD reflections, transmissions and frequency conversion terms as a function of at least the phase of the LO drive signal and optionally also its power with arbitrary incident waves at all the accessible ports. The model assumes the LO is the only signal that drives the mixer into its non-linear domain of operation. This bounds the applicability of the model to the principal or primary behaviour of the frequency translating device. As said, to model this behaviour there is presently no straightforward method available.